A gas turbine adapted for utilizing a methane component contained in the mixture as the fuel thereof by taking the gas into the engine thereof with the concentration of methane being controlled to be lower than the flammable limit has been known as a conventional technology. In the gas turbine of this type, the methane concentration is controlled to be within a range equal to or lower than the flammable limit by mixing a high-concentration methane gas, as needed, with the air containing a low-concentration methane gas. Then, this mixture is compressed by the compressor in order to produce a compressed gas. Thereafter, the compressed gas is combusted by a catalytic oxidation catalytic oxidation in a catalytic combustor in order to produce a combustion gas. In this manner, the turbine of this system can be driven by the so-produced combustion gas. Meanwhile, an exhaust gas discharged from the turbine is fed to a recuperatorrecuperator, whereby the compressed gas introduced into the catalytic combustor from the compressor can be heated (Patent Document 1: WO2004/029433A1). This gas turbine can utilize the low-calorie gas, such as the landfill gas and/or CMM gas, especially the so-called VAM (Ventilation Air Methane) gas, i.e., a ventilation exhaust gas generated from the coal mine, as the fuel thereof. Usually, this VAM gas is discharged or emitted into the air, because the methane concentration of this gas is equal to or lower than 1%, thus insufficient for allowing such a gas to be burned under normal burning conditions. Meanwhile, it is now attempted to get the right to emit carbon dioxide (CO2) by achieving electric generation using the gas turbine that can utilize such a VAM gas as the fuel thereof.
However, in this gas turbine, the compressed gas supplied from the compressor is burned by the catalytic oxidation catalytic oxidation in the catalytic combustor. Therefore, the compressed gas supplied to the catalytic combustor should be heated, usually up to 300° C. or higher, and sometimes up to approximately 500° C., depending on the composition of the catalyst used. In particular, upon starting or during a low load operation of the gas turbine, the heating for the compressed gas performed only by the recuperatorrecuperator tends to be insufficient. Therefore, in such a case, it is necessary to further heat the compressed gas, such as by using an auxiliary heating system, e.g., a pre-combustor or the like. For instance, in the case of the above Patent Document 1, the pre-combustor is provided between the compressor and the catalytic combustor, wherein a propane gas or the like is supplied and burned in this pre-combustor with the pressure of this gas being elevated, thereby to heat the compressed gas before it is supplied to the catalytic combustor. However, in the case of using such an auxiliary heating system, the entire body of the gas turbine should be rather enlarged. In particular, in the case of the above Patent Document 1, it is necessary to further provide a fuel pressurizer adapted for pressurizing the fuel supplied to the pre-combustor up to a certain pressure needed at an outlet of the compressor, leading to substantial reduction of the net power actually used for the electric generation, thus degrading the energy efficiency.
Besides, in the case in which a part of the mixture extracted from the compressor is utilized for cooling high temperature parts, such as the turbine and the like, and/or utilized for shaft sealing, the methane gas contained in such a mixture is discharged to the outside, while being unreacted. That is, the so-called loss of the fuel gas will occur. In addition, such a loss of the fuel gas will also occur when the catalytic power or effect in the catalytic combustor is substantially degraded.